Improving Measurement Accuracy with a Thermal
by Bob Rogers
ITC Senior Thermography Instructor
Thermal imagers show us things we’ve never seen before, but force us to ask different questions. One of the most important is: are you close enough to your targets to get an accurate temperature reading with your thermal imager? We can try and obtain a more accurate temperature by getting closer, but many times this just isn’t possible. Working in and around high voltages means that we have “safe” minimum approach distances that MUST be adhered to. And of course, entering the correct temperature parameters such as background, emissivity, relative humidity and distance into your thermal imager are very important and aid in obtaining the correct temperature value.
For those of us who are scanning electrical substations with thermal imagers, it is very important to obtain the correct temperature. There are several criteria that help us to determine the course of action to be taken based on a temperature. If the measured temperature is not obtained correctly, it could have a disastrous effect on the course of action to be taken on electrical equipment. The electrical anomaly may be interpreted as just a deficiency and not be acted upon as an immediate problem. If the elevated temperature is measured as low by “accident” because the thermal imager is not in focus, in an incorrect range or not close enough, it will cause a similar problem.
The following is an example of a temperature obtained at a substation while I was on a recent trip. The following are thermograms taken with a thermal imager of a connection in the substation.
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Photo taken with a FLIR thermal imager. |
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The connection is at a distance much further than you would like to be, even with a high-tech thermal imager. The temperature measured on the hot electrical connection was 49.9ºC(121.8ºF). The ambient, air temperature was 25ºC(77ºF). This calculates out to be a temperature increase of 24.9ºC(76.8ºF) over the ambient air temperature and 23.9ºC(75ºF) over a similar adjacent electrical connection. Based on a severity criteria from NETA, the “International Electrical Testing Association”, this would be classified as a Deficiency and would be scheduled to be repaired as-soon-as possible.
Examining this from another viewing angle at the same distance as the first thermal image, I attached a 12ºFOV(Field of View) Lens to the thermal imager. This made the thermal imager appear to be now one-half the distance optically to the object.
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Photo taken with a FLIR thermal imager. |
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Now examining the displayed temperature, the photos taken with the thermal imager show a temperature on the hot connection of 67.3ºC(153.1ºF) which is 42.3ºC(108.1ºF) over the ambient air temperature. The temperature of an adjacent similar electrical connection was made and it calculated out to be an increase of 41.3ºC(106.3ºF) over that similar electrical connection. Based on the severity criteria from NETA again, the repositioning of the thermal imager and the use of a narrow field of view (telescopic) lens, brought the camera optically closer and the resulting temperature would now correctly be classified as a Major Deficiency! This electrical connection must be repaired immediately.
So get closer if you can. Take the temperature measurements from different viewpoints. But best of all, use a telescopic lens with your thermal imager that will get you optically closer to the thermal anomaly.
For more information on thermal imagers, please visit http://www.flirthermography.com/cameras/all_cameras.asp.